Amplification circuit and heat sink used with a light emitting apparatus having varying voltages

ABSTRACT

A light emitting apparatus for regulating a current output of an LED at predetermined value with a power source having a wide variety of voltages and chemistries is described. A light emitting diode is electrically coupled to the voltage source. A pulse width modulation controller controls a duty cycle of the voltage applied. A resistor electrically coupled between the voltage source and the light emitting diode is used to regulate output current for the LED. An amplification circuit is electrically coupled to the resistor and the pulse width modulation controller for supplying a feedback voltage to the pulse width modulation controller that is higher than a voltage measured across the resistor.

BACKGROUND OF THE INVENTION

The advent of Light Emitting Diode (LED) technology and it's progressivedevelopment has exposed the shortcomings of Xenon Bulbs in areas ofshort battery life. The increasing use of battery operated tools inworkplaces and the need for energy efficient bright light and increaseddurability led to the design of LEDs in flashlights powered frombatteries.

Currently, tools and flashlights are designed to use just one batteryvoltage. Although two close voltages have been used (9.6 and 12.0),performance (brightness or speed) are still proportional to inputvoltage. Thus, it may be advantageous to have a flashlight that workedwith any battery input voltage, while maintaining even brightnessthroughout its use. This capability offers the user great flexibility inleveraging any of his existing batteries. LED technology also demandsthat the LED be driven at regulated junction temperature conditionsgiving a user years of lifetime and eliminating the need for replacementof Xenon bulbs.

Targeted application scenarios in automotive repair or MRO (Maintenanceand Repair Organizations) require bright light in the range of 120-170lumens. This may be achieved with a series of LEDs driven at 0.35 A by apower source. LED's with forward voltage in range of 3.2-3.8V whenconnected in series represent a total voltage drop of 9V and greater.(i.e. 3 or more LEDs in series, each producing ≈60 lumens). Highly costeffective and efficient Buck operation cannot be incorporated as theoutput voltage will be more than certain battery voltage ranges. Also,optically converging the beams from three different LED's towards asingle beam output may be a challenge.

SUMMARY OF THE INVENTION

The present invention provides in one aspect a light emitting apparatuscomprising a voltage source for supplying an input voltage, a lightemitting diode electrically coupled to the voltage source, and a pulsewidth modulation controller for controlling a duty cycle of the inputvoltage supplied. A resistor is electrically coupled between the voltagesource and the light emitting diode. An amplification circuit iselectrically coupled to the resistor and the pulse width modulationcontroller for supplying a feedback voltage to the pulse widthmodulation controller, the feedback voltage being proportionally changedrelative to a resistor voltage measured across the resistor, a currentthrough the first resistor or a resistance of the first resistor.

The present invention provides in another aspect a light emittingapparatus comprising a housing including a portion defining a thermallyconductive outer surface. A light emitting diode is positioned withinthe housing. An internal heat sink thermally couples the light emittingdiode and the thermally conductive outer surface portion of the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. Included in thedrawing are the following figures:

FIG. 1 is a diagram of a light emitting apparatus according to anexemplary embodiment of the present invention.

FIG. 2 is a diagram showing a portion of the diagram in FIG. 1.

FIG. 3 is a perspective view of a light emitting apparatus according toan exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Current sensing in an LED driver may be based on low side sensing in aresistor connected to cathode of the LED and other end being referencedto ground. The voltage developed (Iload*Rsense) is usually fed back toPWM controller to regulate the duty cycle and control the outputcurrent. Low side sensing, however, is effective only for load currentin the range of 0.2-0.3 A and leads to increasing losses as the loadcurrent increases. At 1 A, power loss can be as high as 20% based onfeedback voltage required by PWM controller. It may, however, bedesirable to have an output current of 1 A because high luminous outputmay be achieved by driving the LED 104 at a current of 1 A having aforward voltage of the LED in the range of 3.2-3.6V, where the forwardvoltage is less than the input voltage condition.

Referring to FIG. 1, a circuit 100 of a light emitting apparatusaccording to an exemplary embodiment of the present invention isillustrated. The circuit 100 includes a voltage source 102 for supplyinga voltage and a light emitting diode 104 electrically coupled to thevoltage source 102. The light emitting apparatus also includes a PWMcontroller 106 for controlling a duty cycle and a resistor R8electrically coupled between the voltage source 102 and the lightemitting diode 104. The light emitting apparatus of the presentembodiment further includes an amplification circuit 108 electricallycoupled to the resistor R8 and the PWM controller 106. The amplificationcircuit 108 supplies an output voltage to the PWM controller 106 that isa given ratio higher than the voltage measured across the resistor R8.

A step-down circuit may be used to regulate the current into the LED104, based on current sense feedback control. The exemplary lightemitting apparatus is desirably configured to use a wide range ofvoltage sources 102 of different chemistries and voltages. A given rangeof voltages may be 5-21V, however, the invention is not limited to sucha range and may be higher or lower. As described above, it is desirableto regulate the output current of the LED 104 to a desired current for avariety of operating conditions, including various voltages. In thepresent embodiment, the desired current is 1 A, but other current valuesmay be utilized. Thus, the light emitting apparatus of the presentembodiment uses the PWM controller 106 to control the duty cycle of thecircuit proportional to the desired current by sensing any increase ordecrease of current as the input voltage value changes over thedischarge cycle of the voltage source 102 or as different voltagesources 102 are used.

The circuit 100, incorporating the current sense feedback control, feedsback to the PWM controller 106 a voltage proportional to the desiredcurrent for the LED 104. The PWM controller 106 is configured toregulate the desired current based on a target feedback voltage, forexample, a feedback voltage of 0.7V at FB pin 4, received from resistorR8. In the present embodiment, the amplification circuit 108 iselectrically coupled to the resistor R8 and the PWM controller 106 andis configured to supply the target feedback voltage to the PWMcontroller 106 (e.g. a 0.7V feedback voltage). In the light emittingapparatus of the present exemplary embodiment, the amplification circuit108 is supplying a feedback voltage that is proportionally higher than avoltage measured across the resistor R8. The proportional increase inthe voltage is dependent upon the desired current, the resistance ofresistor R8 and the target feedback voltage.

In the exemplary embodiment discussed herein, the desired current is 1 Aand the illustrated resistor R8 has a resistance of 0.1Ω. As such, thevoltage measured across the resistor R8 will be 0.1V. If the targetfeedback voltage is 0.7V, the amplification circuit is configured toincrease the measured voltage 7 fold. If the desired current or targetfeedback voltage is changed, or if a different resistor R8 is used, theamplification circuit 108 is modified to provide an appropriateproportional increase.

An exemplary amplifying circuit 108 is illustrated in FIG. 2 andincludes a transistor pair Q1, Q2 arranged as a current mirror,resistors R8, R10 arranged as current divider, and equivalent collectorload resistors R11 & R12, to provide the target feedback voltage to thePWM controller 106 for regulation of the desired current. The currentmirror arrangement holds both transistor Q1, Q2 currents the same andthus both emitters at the same voltage. Starting at common node R8/R10,the voltage dropped across R8 has to be the same as across R10. If 1 Ais desired through R8, a ratio of 1000:1 will allow just 1 mA to beconducted through Q1. This allows for a very small value to be chosenfor R8 (0.1Ω). In operation, a higher ratio, e.g. 1300:1, may be used tocompensate for PWM controller impedance. R11 and R12 are desirablyequivalent and have a value chosen to offer the correct feedback voltageto the PWM controller 106 at the Q1 current level chosen.

The values for the electrical components shown in FIG. 2 are forsupplying a feedback voltage of 0.7V to the PWM controller 106 for avoltage of 0.1V measured across the resistor R8 having a resistance of0.1Ω. As will be recognized by one skilled in the art, the amplificationcircuit 108 allows for the use of a resistor R8 having a lowerresistance and thereby a lower loss. In the illustrated embodiment, aresistance of 0.1Ω leads to a loss of only 0.1 W for the total outputpower of 3.6 W. Thus, a higher energy transfer ratio is achieved, suchas, for example, in the power range of a 4 W LED at wide variable inputvoltage range. The lower heat dissipation lost for current measurementmay allow a longer run time for the light emitting apparatus.

The values for the electrical components shown in FIGS. 1 and 2 are,however, only exemplary. It is contemplated that other values may beused. It is also contemplated that different configurations ofelectrical components may be used for supplying a target feedbackvoltage to the PWM controller that is higher than a voltage measuredacross the resistor R8. It is also contemplated that other amplifyingdevices (e.g., an operational amplifier) may be used for supplying atarget feedback voltage to the PWM controller 106 that is higher than avoltage measured across the resistor R8.

The exemplary embodiment of the present invention drives an LED at aload current of 1 A to achieve the same lumen output as three or morelower current driven LEDs. The forward voltage of LED may operate at3.7V and 1 A of current. Thus, a highly efficient Buck operation can beincorporated as the output voltage is always less than the range ofvoltages supplied by the voltage source 102. The PWM controller 106controls the duty cycle to regulate the current to 1 A based on inputvoltage and power available. The PWM controller 106 may be configured toshut down as the input voltage goes under a threshold value (e.g. 4.2V),thereby providing an under voltage cut off feature. Thus, the batterysource will be protected from over discharge failure or damage.

The LED's photometric output increases proportionally with the current,as long as junction temperature is maintained at permissible levels. AnLED may characteristically show variable photometric output with achange in junction temperature. Elevated temperatures may also lead toaccelerated LED degradation. Thus, it may be desirable to maintain andcontrol the junction temperature.

Referring to FIG. 3, a light emitting apparatus according to anotherexemplary embodiment of the present invention is shown. The exemplarylight emitting apparatus includes a housing 300 which houses the LED inalignment with a lens 306. The illustrated housing includes first andsecond housing components 302 and 304. While two housing components areillustrated, more or fewer may be utilized.

In the present embodiment, the LED is mounted inside of the housing 300on an internal heat sink (not shown) that is thermally connected to aportion of the outer housing 300. The internal heat sink may be, forexample, a metal core board, but other structures configured to transferheat from the LED to the portion of the outer housing 300 may beutilized. The portion of the outer housing 300 to which the internalheat sink is thermally connected is desirably made of a material whichalso transfers heat, such that the portion of the outer housing 300provides a large heat dissipating outer surface exposed to atmosphere.

The portion of the outer housing 300 to which the internal heat sink isthermally connected may, for example, be a finished Aluminum surface asa part of housing 300. Aluminum includes properties (i.e. strong andlight) that provide for an elegant, sleek design that dissipates heatfor controlling the temperatures of the LED. It is contemplated,however, that other materials may be used for the thermal dissipatingsurface area. Additionally, it is contemplated that one or both housings302, 304 may include the portion of the heat sink.

This exemplary light emitting apparatus helps in improved thermalperformance by “conductively” transferring heat from the LED junction toan internal heat sink and from an inner portion of the heat sink to anouter heat dissipating surface area. The exposed surface to atmospheremay also result in increased heat dissipation through radiation. Thus,this design innovatively maintains temperature of a high power LED atsafe permissible levels even when operated at high currents.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed is:
 1. A light emitting apparatus comprising: a voltagesource for supplying an input voltage; a light emitting diodeelectrically coupled to the voltage source; a pulse width modulationcontroller for controlling a duty cycle of the input voltage supplied; afirst resistor electrically coupled between the voltage source and thelight emitting diode; and an amplification circuit electrically coupledto the first resistor and the pulse width modulation controller forsupplying a feedback voltage to the pulse width modulation controllerthe feedback voltage being proportionally changed relative to a resistorvoltage measured across the first resistor, a current through the firstresistor or a resistance of the first resistor, wherein theamplification circuit includes a first transistor and a secondtransistor, and wherein the amplification circuit includes a secondresistor coupled in parallel with the first resistor for dividing aninput current between the first and second transistors.
 2. A lightemitting apparatus comprising: a voltage source for supplying an inputvoltage; a light emitting diode electrically coupled to the voltagesource; a pulse width modulation controller for controlling a duty cycleof the input voltage supplied; a first resistor electrically coupledbetween the voltage source and the light emitting diode; and anamplification circuit electrically coupled to the first resistor and thepulse width modulation controller for supplying a feedback voltage tothe pulse width modulation controller the feedback voltage beingproportionally changed relative to a resistor voltage measured acrossthe first resistor, a current through the first resistor or a resistanceof the first resistor, wherein the amplification circuit includes afirst transistor and a second transistor, and wherein the amplificationcircuit includes a first load resistor and a second load resistor, thefirst load resistor coupled between the first transistor and the pulsewidth modulation controller and the second load resistor coupled betweenthe first resistor and the first transistor.
 3. The light emittingapparatus according to claim 2, wherein the amplification circuit isconfigured to control load currents through the first and second loadresistors to be substantially the same and load voltages across thefirst and second load resistors to be substantially the same.
 4. Thelight emitting apparatus according to claim 2, wherein the first andsecond load resistors have substantially the same resistance.
 5. A lightemitting apparatus comprising: a voltage source for supplying an inputvoltage; a light emitting diode electrically coupled to the voltagesource; a pulse width modulation controller for controlling a duty cycleof the input voltage supplied; a first resistor electrically coupledbetween the voltage source and the light emitting diode; and anamplification circuit electrically coupled to the first resistor and thepulse width modulation controller for supplying a feedback voltage tothe pulse width modulation controller the feedback voltage beingproportionally changed relative to a resistor voltage measured acrossthe first resistor, a current through the first resistor or a resistanceof the first resistor, wherein the amplification circuit includes afirst transistor and a second transistor, and wherein the amplificationcircuit is configured to control transistor currents through the firstand second transistors to be substantially the same and transistorvoltages across the first and second transistors to be substantially thesame.